A conventional tensioner includes a housing with a plunger-accommodating hole, a plunger protruding from the plunger-accommodating hole and slidable therein in so that it can advance and set back in an advance/set back direction. The tensioner also includes a spring for biasing the plunger in the advancing direction. The plunger moving in the advancing direction applies a tension to a transmission medium by urging a movable guide against the transmission medium as disclosed in U.S. Pat. No. 7,442,138, granted Oct. 28, 2008.
Another known tensioner, disclosed in Japanese laid-open Patent Application 2008-144840, published Jun. 26, 2006 includes a plunger formed by a deep drawing process.
In order to make the wall of a hollow plunger thin and reduce the weight of the plunger, a material having favorable malleability so that it can be forged is preferable. The material should also be relatively soft material so that it can be cut easily. Accordingly, carbon steel is ordinarily used as the material for the plunger. However, the use of carbon steel has a drawback in that treatments such as heat treatment and rustproofing are required to impart abrasion resistance to the material after forming the plunger by forging or cutting.
A very small radial gap is formed between a circumferential wall surface of the plunger-accommodating hole of the housing and the outer circumferential surface of the plunger to allow the plunger to slide in the advancing and setback directions. In the operation of the tensioner, because of friction between the plunger and a movable guide, a force imparted to the plunger by the movable guide can cause the plunger to incline within a range dependent upon the size of the radial gap.
As shown in FIG. 6, in a tensioner 500, if the outer circumferential surface of the rear portion of the wall 511 of the plunger 510 has a C-chamfered portion 513, and the plunger 510 is located at its most advanced position the plunger can become inclined in such a way that the corner-shaped boundary 517 between the cylindrical part 512 of the outer wall of the plunger and the chamfered portion 513 comes into contact with the circumferential wall surface 515 of the plunger-accommodating hole 514. When such contact occurs, contact pressure, i.e., Hertzian stress, at the boundary 517, and in the adjacent parts of the C-chamfered portion 513 and the cylindrical part 512, increases.
As shown in FIG. 7, if the outer circumferential surface of the rear portion of the wall 611 of the plunger 610 has an R-chamfered portion 613, the surface of the R-chamfered portion is substantially continuous with the cylindrical portion 612 of the plunger. If the plunger 610 is at its most advanced position the plunger can become inclined as in the case of the plunger of FIG. 6, and the contact pressure between the wall surface 615 and the boundary portion 617 increases causing Hertzian stress in the boundary portion 617 as well as in the adjacent parts of the R-chamfered portion 613 and the cylindrical portion 612.
The above-described increases in contact pressure result not only in accelerated wear of the plungers 510 and 610 but also in increased friction between the plungers 510 and 610 and the wall surfaces 515 and 615 of the plunger-accommodating holes thus reducing the ability of the plungers and 610 to slide in the advance/setback directions.
Accordingly, there is a need for a tensioner in which the durability of the plunger is improved, in which the plunger responds more rapidly to changes in tension in a transmission medium, in which the plunger can be produced at reduced cost by a deep drawing process, and in which the contact pressure at the location at which the plunger contacts the wall of the plunger-accommodating hole is reduced.